Off-highway work vehicles such as loader-backhoes utilize a variety of hydraulic system architectures. One common arrangement is called an “open center” architecture. In a typically open center systems, a constant displacement pump such as a simple gear pump is used as a primary source of hydraulic fluid for the various hydraulic devices in the system. The pump provides a constant flow rate of hydraulic fluid through the system that does not vary with time. The control valves function by restricting this hydraulic fluid flow through the pump and providing an alternative path into the actuator to be moved. The pump responsively raises the pressure in its outlet line (i.e. the main hydraulic supply line) sufficient to maintain a constant flow rate through the pump. In an open center system, generally speaking, the flow rate through the pump is constant and the load on the pump and engine varies with changing head pressure.
In another common arrangement, called a “closed center” architecture, a variable displacement pump (such as a de-stroking piston pump) is provided that is configured to maintain a relatively constant output pressure regardless of the flow rate over time. The various control valves function by connecting the devices they control to the output of the pump. When the control valves provide this alternative flow path, the output pressure tends to drop and the control circuit for the pump compensates by increasing the specific displacement of the pump. When the specific displacement is increased, the pressure is restored to its design output pressure. When the operator closes the valve that conducts fluid to the desired device, the pressure increases in the system and the control circuitry for the hydraulic pump responsively reduces the specific displacement of the pump (i.e. the pump is “destroked”). In a closed center system, generally speaking, the pressure provided by the pump is constant and the flow rate is varied as necessary to maintain a constant head pressure.
Some systems are hybrids of both open center and closed center components. In these systems, some control valves are configured to operate as open center valves and some control valves are configured to operate as closed center valves. In these hybrid systems, the pump is an open center (i.e. constant displacement) pump. This mandates that one modify the closed center valves for use in an otherwise open center system. The closed center valves must be coupled to the hydraulic supply at a point downstream from the open center components in order to operate properly.
The modifications in hybrid systems include an inlet compensator at the inlet of the closed center valves that directs hydraulic fluid either to the closed center function when the closed center valves are selected, or directs it to the tank when the open center valves are used. The inlet compensator requires a constant pressure differential be established to work correctly but increases average working pressure, higher neutral standby pressures, more component complexity/cost to make the system perform correctly, increased fuel consumption, etc.
What is needed is a system for directing fluid flow to both open center and closed center components that reduces the losses in current subsystems. What is also needed is a system that eliminates the need for an inlet compensator. It is an object of this invention to provide such a system.